Numerous types of water filtration systems are commercially available. Traditionally, beds of loose carbon particles were used for removing metals and/or organic materials from water. Composite blocks can be made from combinations of sorptive materials, such as adsorbent activated carbon, and polymeric binders, such as ultra high molecular weight polyethylene (UHMW PE), that have been sintered together under conditions of heat and pressure and are useful in water filter technology. Carbon block technology, for example, provides comparable functionality to loose bed carbon particles without the particle shedding or taking up too much space. With carbon block technology, pressure drop across the block can increase as a result of increasing quantities of adsorptive materials. Moreover, exposure of carbon blocks to heat and pressure can limit the types of adsorptive materials available for use in the blocks. Historically, carbon block technology was generally precluded from using adsorptive media that is sensitive to thermal degradation, such as ion exchange resins.
Improved performance for ion exchange, however, has potential application to a wide variety of fluid purification applications. One example is a growing market for scale control systems for water used in the food and beverage industries. Current systems rely on disposable (non-regenerable) fixed beds of cation exchange resin to remove hardness. Currently, limitations include relatively short service life, relatively high pressure drop and low flow rates. Development is needed to improve kinetics and to increase treatment capacity per unit volume of media. Another application is for residential softening applications, which although typically regenerable, there is a desire to make them smaller while maintaining capacity and treatment rate. Similarly, there is also a need to improve capacity and reduce size of systems for filtration of photoresist compositions and high-purity chemicals as may be used in the electronics manufacturing industry.
Literature indicates that performance of fixed beds of cation exchange resins is often limited by film diffusion or by intraparticle diffusion. Most commercial resin particles are relatively large (0.6-0.9 mm) Although it can be expected that use of smaller particles would generally improve kinetics, the understanding has historically been that smaller particles would lead to an even higher pressure drop.
A concern with respect to immobilizing ion exchange resin is that literature and resin vendor data indicates that the media is not stable at the high temperature used for melting binders such as ultra high molecular weight polyethylene. Data provided by cation exchange resin suppliers indicate that functionality of resin can be reduced at temperatures above 120° C.
Another concern in molding ion exchange resin is that the material swells on wetting and swells on exchange (as much as 90%), which could cause serious problems with integrity of media composites or cause pressure drop problems.
There is an ongoing need to provide water filtration systems that use ion exchange resin in composite form that provides excellent ion exchange kinetics without deleterious effects on pressure drop and equilibrium capacity.